Vertex coordinate monitoring during Transform
If you want to bring a vertex of one mesh to a particular vertex of another mesh do this:


select the first object, switch to edit mode, deselect all vertices and select the one to which you want to bring the other vertex. 
Switch to object mode
Similarly select the other object and repeat the above steps to selecting the node you want to be moved
Execute the following script replacing the name 'Cube1' with the reference object and 'Cube2' with the object whose vertex you want to move:

import bpy
obj1 = bpy.data.objects['Cube1'] 
obj2 = bpy.data.objects['Cube2'] 
co = obj1.matrix_world * [v for v in obj1.data.vertices if v.select == True][0].co
[v for v in obj2.data.vertices if v.select == True][0].co = obj2.matrix_world.inverted() * co